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'[EE]: need RS232 transceiver chip'
2000\07\25@105717 by Brian Aase

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No doubt lots of you know the answer to this...
I'm looking for an RS232 transceiver chip that more or
less amounts to a 1488 and 1489 all on one single IC.
(space is tight, and would like to avoid the need for 2 chips.)
I want to stay away from the MAX232 et al because they
all contain an oscillator that adds to the overall EMC
noise.  I already have plus/minus 12 volts on the PCB.
All I have found are some obsolete parts from Mot and TI.
Seems like there must be something in current production .
Suggestions?

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2000\07\25@114925 by Dan Michaels

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At 07:55 AM 7/25/00 -0700, you wrote:
>No doubt lots of you know the answer to this...
>I'm looking for an RS232 transceiver chip that more or
>less amounts to a 1488 and 1489 all on one single IC.
>(space is tight, and would like to avoid the need for 2 chips.)
>I want to stay away from the MAX232 et al because they
>all contain an oscillator that adds to the overall EMC
>noise.  I already have plus/minus 12 volts on the PCB.
>All I have found are some obsolete parts from Mot and TI.
>Seems like there must be something in current production .
>Suggestions?
>

You might check what Dallas Semi has available.

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2000\07\25@132430 by Morgan Olsson

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At 07:55 AM 7/25/00 -0700, you wrote:
>No doubt lots of you know the answer to this...
>I'm looking for an RS232 transceiver chip that more or
>less amounts to a 1488 and 1489 all on one single IC.
>(space is tight, and would like to avoid the need for 2 chips.)
>I want to stay away from the MAX232 et al because they
>all contain an oscillator that adds to the overall EMC
>noise.  I already have plus/minus 12 volts on the PCB.

What about using a MAX232 etc anyway, but omit the switching caps, and feed the + and - 10Volt terminals with + and - 12V?

Still a small internal osc running, but no power through cap switches.

/Morgan

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2000\07\25@143145 by M. Adam Davis

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Any particular reason you couldn't use 3 or 4 transisters and a couple
of resisters to do this?

It would cost you less than nearly any single chip solution for the
part, though assembly might be a tad bit more expensive.  It would take
up less space on a board than a 14 pin DIP, especially if you go with
surface mount components.

Actually, you should be able to get by with only two transisters and a
few resisters.  This should be cheaper than buying a seperate chip no
matter what assembly technique you use.

-Adam

If you can't do it with duct tape, baling wire and a swiss army knife,
it ain't worth doin'.

Brian Aase wrote:
{Quote hidden}

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2000\07\25@150504 by Oliver Broad
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I believe there's something in the Dallas range, took it's -ve from the
incoming data line (like a mouse).


{Original Message removed}

2000\07\25@215813 by McMeikan, Andrew

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that would be the DS1275 (8pin), there is a newer part (DS275 ?) that does
the same thing, I seem to remember that you could also feed it the -ve if
you had it available rather than just parasite the power.

       cya,    Andrew...

> {Original Message removed}

2000\07\26@003941 by Dwayne Reid

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At 07:55 AM 7/25/00 -0700, Brian Aase wrote:
>No doubt lots of you know the answer to this...
>I'm looking for an RS232 transceiver chip that more or
>less amounts to a 1488 and 1489 all on one single IC.
>(space is tight, and would like to avoid the need for 2 chips.)
>I want to stay away from the MAX232 et al because they
>all contain an oscillator that adds to the overall EMC
>noise.  I already have plus/minus 12 volts on the PCB.
>All I have found are some obsolete parts from Mot and TI.
>Seems like there must be something in current production .
>Suggestions?

I have a dumb suggestion that I have not tried out myself but *might* work:
use a MAX232 but don't use the built in charge pumps.  The oscillator will
still run but it is the charging of the capacitors that causes most of the
EMI.  Leave off the 2 charge pump capacitors and feed +12V into pin 2, -12V
into pin 6.  You will need small bypass caps (100n) from those pins to
ground, as well as from VCC (pin 16) to gnd.  The neat thing is that you
can delete all 4 large caps from the charge pump.

Again, I have *not* tried this myself and do *not* know the consequences of
power supply sequencing.  But it should work and uses readily available parts.

dwayne



Dwayne Reid   <spam_OUTdwaynerTakeThisOuTspamplanet.eon.net>
Trinity Electronics Systems Ltd    Edmonton, AB, CANADA
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2000\07\26@113736 by Dan Michaels

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>At 07:55 AM 7/25/00 -0700, Brian Aase wrote:
.......
>>I want to stay away from the MAX232 et al because they
>>all contain an oscillator that adds to the overall EMC
>>noise.
.......


Regarding EMI generated by these chips, I have not specifically
checked the MAX232, but have extensively used the 766x-type
negative voltage converter chips. Both use similar switched
capacitor charge pumps. The 766x generate significant EMI - probably
much more than the MAX232 since they pump more current - but
if you use a ground plane under the chip, most of the noise
is squelched.

These facts - re EMI - are not well publicized in the datasheets
AFAICT. I found this out the hard way by using a 766x [originally
with no gnd plane] to drive a high sensistivity amp that was located
1" away.

best regards,
- Dan Michaels
Oricom Technologies
http://www.sni.net/~oricom
==========================

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2000\07\26@134715 by Peter L. Peres

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A cheap suggestion that has passed most tests here: LM339 with a 2.5V
reference (10K+10K from 5V to GND) for receive, and pullups on the
outputs. You can also use a LM393 (one pair) in the same way. The input
(RxD) line is through a 4K7/47K to gnd to stay in the CMMR of the chip.
Pullups depend on the speed.

Peter

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2000\07\27@014702 by Vasile Surducan

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On Wed, 26 Jul 2000, Dan Michaels wrote:

> >At 07:55 AM 7/25/00 -0700, Brian Aase wrote:
> .......
> >>I want to stay away from the MAX232 et al because they
> >>all contain an oscillator that adds to the overall EMC
> >>noise.
> .......
>
  Max 232 family can be supplied from +5V and other +/- source if
connections to oscilators are interrupted. Without oscillator load the EMC
noise must
be lower than usual. All the problem may occure is to not overload the
comunication pins, the chip will become hot...
  Vasile

{Quote hidden}

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